Rivers 2

As you go downstream...

V-shaped valley with interlocking spurs and a narrow channel and low discharge with turbulent flows.

Larger bedload/boulders come from freeze thaw and weathering from valley sides. There are also boulders on valley sides known as scree.

Vertical erosion taking place which can cause potholes to form. Waterfalls are also found here along with rapids.

Low hydraulic radius and velocity causing in-efficient channel.

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As you go downstream...

Middle course

Gently sloping sides, wider channel width and depth.

More lateral erosion taking places and the spurs are cut back (become truncated), first sign of meanders appear.

Discharge is greater; there is smaller more rounded bedload as erosion and attrition take place.

The floodplain is more developed.

There is laminar flow and velocity increases.

The hydraulic radius increases which leads to a efficient channel.

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As you go downstream...

Lower course

Deeper and wider channel with very gentle valley sides.

More sinuous with increased developed meanders (oxbow lakes may form)

Very wide floodplain occurs.

Higher discharge with reduced competence, fine bedload sediments.

There is laminar flow and increased velocity.

Increase hydraulic radius so a more efficient channel.

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River processes - EROSION

Erosion definition: The wearing away of rocks and material due to the effects of wind, ice or water.

3 types of erosion: Hydraulic action, Abrasion, Attrition.

Hydraulic action

This is when the sheer force of water scrapes away the bed and banks of the river. It can also be referred to when water forces it way into cracks in the banks and overtime the pressure causes the crack to widen and collapse.

Abrasion

This is when material (.e.g rocks) scrape, scour and rub against the banks and bed causing them to wear away.

Attrition

This is when particles in the river collide causing them to break into smaller pieces. This makes the material more rounded, smaller and smoother.

Corrosion

When the water is slightly acidic, it starts to dissolve the river's bed and banks when they are made up of rocks (e.g. limestone)

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River processes - TRANSPORTATION

Transportation defintion: The movement of material following its erosion.

A decrease in the gradient of the river channel causing the energy of the river to reduce.

A lower discharge in the river resulting in a inefficient channel.

If the river has flooded, it leads to water speading out onto the floodplan which means the rivers energy is spread out.

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Hjulstrom curve

The relationship between erosion, transportation and deposition.

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Hjulstrom curve

Sand has the smallest entrainment velocity.

For erosion of the particles to take place, the velocity needs to be higher than the entrainment velocity.

The entrainment line shows the velocities needed to pick up particles of different size. In general as particle size increases the entrainment velocity also increases.

The fall velocity line shows the velocity at which different sized particles are deposited. E.g. for boulders it only takes a slight drop in velocity for them to be deposited.

As velocity continues to decreased even smaller particles will be deposited except for clay.

A particle needs to be entrained first and then transportation takes place even though velocities are lower.

The clay particles are never deposited in the river because they are always suspended as they're so small and light.

Deposition of clay only occurs when the river flows into the sea at the river mouth. Here salt water causes clay particules to focculate.

The smallest particles (E.g. clay) require just as high a velocity as boulders to be entrained. This is because clay has a flat surface area so it's difficult to entrain.

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Laminar and turbulent flow

The two types of river flow are laminar and turbulent flow. A rivers flow is related to its velocity.

Laminar flow (smooth flow):

High velocity

Less friction

Smaller bedload

Turbulent flow (swirling, chaotic flow)

Vertical and horizontal eddies (an eddy is a river feature caused by an obstruction to the waters flow, forcing it to flow in the opposite direction. Eddies can be known to create small whirpools on the surface)

Low velocity

More friction

Large, rough bedload

Isovels are a type of isoline (lines which join points of equal value e.g. isobars).

Isovels join up equal points of equal velocity

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The factors of velocity

Cross sectional area (XSA) : Width x mean depth

Velocity: Speed of flow of a river, measured in metres per second (m/s)

Discharge: Volume of water at a given point at a given time. Measured in cums (m^3/s)

Wetted perimeter (WP): Refers to the total length of an area of a river's bed and banks that is in contact with the water when viewed in cross section. A larger WP means more fricition between the water and the channel and bed and banks.

Hydraulic radius (HR): XSA/WP HR is the measurement of a river's efficiency.

Linked factors:

Increased HR = Increased velocity - positive relationship

Decreased HR = Increased WP - negative relationship

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Waterfalls

Definition: A steep fall of river water where there is a change in the level of the river bed

Description:

Shape - Come in many different shapes and each is unique. Typical shape is to have a band of more resistant rock that stands about the less resistant rock and so as the channel flows downstream, it's forced to drop to a lower level.

Location - Upper course of the river as there's more vertical erosion.

Features - Plunge pool at the foot of waterfall & as waterfall retreats a new plunge pool is formed. As the resistant rock is undercut and the waterfall retreats a steep sided gorge is left behind. A splash zone is usually found behind the plunge pool. The long profile will be graded overtime. Potholes may form on the river bed of the plunge pool.

Explanation of waterfall formation

A waterfall will occur where there's a sudden change in the gradient of the river as it flows downstream. They occur due to differentiated erosion - as a river channel flows over different bands of rock with varying resistance the rate of erosion will differe. The less resistant rock will be eroded more quickly. Therefore, leaving the more resistant rock at a higher level, creating a step in the river. Over time the hydraulic action of the river and abrasion will enlarge this step, creating a waterfall.

Velocity increases asd the water loses contact with the river bed, meaning that there is high turbulence in the water when it reaches the lower step in the stream. Turbulent flow with eddie currents and splashback undercut the cap of resistant rock. The less resistant rock underneath is eroded by chemical erosion and biological weathering, caused by the roots of vegetation which grows in the wet environment which is ideal for many plants. Also the wet environment around these rocks means FTW ic common, as the state in which the water exists changed the less resistant rock is weakened. The layers of weaker rock are excavated more quickly by this action meaning the harder rock may be undercut and eventually collapse.

This leaves angular, resistant material in the plunge pool which will combine with hydraulic action to continue to further deepen and undercut through the process of abrasion. As this is a repeated process the waterfall willl retreat (by headward erosion) upstream leaving behing a deep plunge pool and a steep sided gorge with U-faced sides which represents the length of the retreat. Waterfalls are an example of vertical & headward erosion.

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Rapids

Defintion: Sections of fast flowing water that develop where there is a local steepening of the gradient over the course of a river.

Description/explanation:

Common in the upper course of a river as a result of the river cutting down rapidly in a localised area.

Have much steeper gradients than other parts of the river, as the slope angle increases energy levels also increase and therefore rapids are a section of fast flowing water with high velocity. However, the water in rapids is highly turbulent due to large bedload in the channel and steep steps (inefficient channel here). The velocity of a river at rapids is not faster but not efficient in its flow. Due to the roughness of the channel, flow is turbulent and known as white water. This white water is thrown up and characterised by eddie currents.

Rapids usually occur due to the differences in geology over which the river channel flows. Alternating bands of softer and harder rock will lead to alternating rate of erosion. Softer rock is eroded quicker (less resistant) by hydraulic action & abrasion caused by bedload. The harder, more resistant rock is not eroded as quick & therefore the heigh of different sections differ (so there's an uneven gradient)

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Potholes

Definition: Fairly circular hollows or depressions on the channel bed drilled by turbulent high velocity water loaded with pebbles.

Description

Shape - Potholes are circular shaped; cylindrical holes

Example - River Wharf, Yorkshire (The Strid at Bolton Abbey)

Location - Upper courses of river due to increased turbulence and erratic swirling of the water

Features -

Some exposed above river level

Some can become angular & uneven which causes flow of water to be turbulent. over them. However, some potholes are smooth and round.

Size - Varied, some merge and become larger. Couple of cm an extend to 1-2m in diameter.

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Explanation of potholes formation

During periods of high discharge the turbulent water in the upper course can create swirling/eddying motion. The pebbles (some of the bedload) can become trapped in these slight ho9llows and vertical eddies in the water are strong enough to allow the sedinment to grind a hole into the rock by abrasion (corrasion). This is known as pothole drilling. Hydraulic action may also be present in the formation of potholes as the force of the water may increase their size. This may be particulary evident at waterfalls where there is more force from the water at plung pools. It's most effective at times of high velocity and discharge. As verticle erosion is the dominant process, potholes tend to form well above base level where potential evenergy is high. These trapped particles then either dislodge themselves after the potholes become too big to contain them or are smoothered and worn down by the process of attrition as the river continues to flow over them. Neighbouring potholes may join together creating sudden and deepening of the channel.

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Braided channels

Occur in rivers with high sediment load, especially suspended sediment & have a smaller HR )shallower water spread out over a fairly wide river channel) found in meandering parts of river in early stages where erosions high due to fast flowing water

Velocity of the river is lower as the WP is larger; meaning more friction between the water and bed.

In accordance with Hjulstrom's curve, when the velocity decreases in a river, material is deposited in size order . This leads to eyots (Small islands) being formed in the middle of the river, made up of the channels, so now the river follows several different channels instead of one.

There's turbulent flow since the different channels interchange and the water flows over angular bedload or over dipping bands of more resistant rock.

When the river floods, or velocity increases due to increased discharge in times of high rainfall or snowmelt, then the eyots will be eroded or entrained and new ones form later when velocity drops again.

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Floodplains

The part of a valley floor which a river may flood from time to time. When the river floods it deposits a layers of silt, this will then gradually built up to be the height of the floodplain. The edge of a floodplain is usually marked by a slope; this is known as the bluff line. Floodplains can be found throughout a rivers valley.

The coarsest material us deposited closest to the river and as the distance increase from the channel is smaller progressively smaller sized particles are deposited on the floodplain. The Hjulstrom's Curve suggests that material is deposited in size order from largest to smallest. This is because the rivers velocity is at its highest at source of the so can entrain bigger particles. Once entrained they can be transported downstream. The grading of the sediment is the particles are smaller the further away from the river because of a decrease in efficiency due to an increased in the WP. This then leads to a decrease in velocity.

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Floodplains

Deposition of silt: Silt is deposited on the floodplain because the XSA has a slight increase. However, the WP increases a lot. This results in a huge increase in friction. Therefore velocity drops. Material is deposited as velocity drops because there's not enough energy to transport it along the floodplains. This is why the larger more course material is deposited first, forming levees and the smaller, lighter material is dropped on the floodplain.

Bluff line: The bluff line is the bottom of a steep slope which has occured from lateral erosion. This can create the bluff line at the edge of a floodplain.

Creation: The bluff line is created from the erosion that takes place in the outside bend of a meander. Meander migration then forms the formation of a bluff line.

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Meanders

Meanders are long sinuous, winding bends in a river (Shrewsbury is built around a meander in R. Severn)

Location - middle and lower courses of the river because of more laterla erosion. Most sinuous in the lower course.

Meanders can create different features on a rivers course such as:

Point bars (inner bend); River cliff (Outer bend); Asymmetrical channel cross section because of deeper outer bend. ; Oxbow lakes can form if the meander becomes very sinuous & the neck is cut off or meander scars can form if the oxbow lake silts up (Dries up). 'Thalweg' is the fastest flow of the outer bend. This is becayse there's less water in the XSA in contact with the WP (higher HR) so less friction to slow it down, so the river has more energy to erode here. Flow is slowest on the inner bend. When discharge is low, the river's velocity slows, so deposition occurs and riffles are formed. The water is forced to move around these deposits & this creates pools - areas where velocity is high (river gains energy) so erosion is also high. This creates shallow riffles and deep pools in the river, this means the Thalweg (fastest flow of water) is directed towards the bank. Centripetal force drives the water into the outside bank and hydraulic action takes place. Abrasion also occurs if the water is carrying particles such as in suspension. Helicoidal flow (type of secondary flow in a cork screw motion) can create point bars, where eroded material is carried to the inner bend from the outer bend in the meander and deposited there. Overtime the meander will move downstrea, amd sideways due to this outside bend erosion and inside bend deposition, this is called Meander migration.

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Oxbow lake

An oxbow lake forms when the neck of a meander continues to be eroded more and more over time by hydraulic action, abrasion and corrosion, this makes the neck very narrow. Usually, during a flood the neck is broken through and the river then takes the shortest route - which is the new route.

The oxbow lake is then eventually separated from the river due to deposition; it is then usually left where it is.

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Deltas - Depositional feature located at mouth

Formed when deposition exceeds sediment removal. Form in coastal areas that have weak currents as there's less transportation of sediments. In areas that have high tidal rang and high fetch. When the sediment load of a river itself is high.

Examples:

Cuspate deltas: Trianular, pointing out from the mouth. Material is spread out evenly on either side. E.g. Tibet

Arcuate deltas: Rounded edge at mouth of river. Land arches out to the sea and splits many times. Occur when the sediment is course grained E.g. Nile.

Birds food deltas: Dis-tributaries stretch out, surrounded by sediment, forming a bird's foot like shape. The river splits on its way to the sea. E.g. Mississippi.

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Deltas

How do they form

When fresh water (River) meets salt water (Sea). Firstly, the river loses energy and competence when the velocity falls, which causes it to deposit material. Also, when the fresh and salt water meet, an electric charge is produced and clay particles combine (flocculation) and settle on the bed. Deltas are rare because they require calm areas of sea where there's a short fetch so that there are no strong waves and no longshore drift.

Topset beds: Heavier particles (sand & fine gravel)

Foreset beds: Medium grained particles (fine sand & silt)

Bottomset beds: Very fine particles (fine silt and clay)

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River profiles - valley long profiles

Potential energy of the river is due to altitude or posdition above base level (Stored energy). Kinetic energy is due to discharge, velocity and the gradient in which waters flowing.

Shows: Changes in the altitude of a rivers course from source mouth, shows gradient - steeper in UC & gentle in LC.

Valley long profiles:

Graded profiles.... refers to the idealised long profile where the gradient gradually decreases as you move downstream, shown as a smooth curve.

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River profiles - valley long profile

Ungraded profiles...in reality irregularities occur along the long profile.

Shows: Changes in the altitude of a rivers course from source mouth, shows gradient - steeper in UC & gentle in LC.

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River profiles - valley cross profiles

Main proccesses which influence the cross valey shape:

Shows: A view of the valley from one side to the other.

Slope retreat is due to weathering and mass movement. FTW is where rain enters joints in rock, at night if temp is below 0 the water freezes and expands by 9%. In the day it thaws, the diurnal temp fluctuate around 0 degrees.

River processes - channel cross profiles

Shows: A diagram of the channel showing the characteristics from bank on one side to the other side.

VALLEY LONG PROFILE

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Process and impact of rejuvenation

River rejuvenation is where the river erodes down to a new base level. This is caused when the sea level falls and so the river gains potential energy thus having more erosive power and so can easily erode downwards to a new level.

Stage 1) The river is at the base level at the mouth.

Stage 2) The sea level drops meaning the river is left at a height and so a waterfall at the coast is formed.

Stage 3) Erosion works upstream forming a waterfall and a gorge as the waterfall retreats.

Stage 4) The waterfall retreats further upstream until it becomes a rapid and the mouth is left at the new base level.

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Rejuvenation

At stage 2 the sea level drops, this could be due to many reasons:

A climatic event such as an ice age can freeze the water locking it away and lowering sea levels.

Climage change can melt ice on the land causing more water to be in the so sea levels rise.

Tectonic activity such as mountain building can mean a relative moment in relation to the sea level of the land and so the sea level can appear to fall.

River terraces

River terraces are former floodplains which have been left about the level of the modern floodplain because the river has had increased vertical erosion due to a rise above sea level leading to an increase in potential energy. Some of the original flooplain will not be acted upon by erosion and so it will be left higher than the new floodplain. Eventually several of these terraces can build up showing that the river has been acted upon by a lot of erosion and so the sea level must have varied for there to be lots of vertical erosion. When the sea level rises, material is deposited on the floodplai as the river has less potential energy and this is called aggradation.

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Incised meanders

Formed when a river keeps the same course for a long period of time and so vertically erodes downswards creasting a deep widening valley with very steep sides. When the land rises or the sea level falls the potential energy of the river increases and so it can erode vertically downward at a faster rate. There's 2 different types of incised meanders:

Entrenched meander: Formed quickly by vertical erosion and so are symmetrical with a gorge like appearance.